1. Field of the Invention
The present invention generally relates to an electric motor drive circuit. More particularly, the invention relates to an electric motor drive circuit for electric power steering systems for vehicles of the type in which auxiliary steering torque is produced by means of a steering servo device using an electric motor.
2. Description of Relevant Art
In view of problems in hydraulic power steering systems such as that the structure thereof is complicated, in recent years a variety of electric power steering systems for vehicles have been proposed.
As an example, one of them is disclosed in UK Patent Application Publication No. GB A No. 2132950, published July 18, 1984.
According to this UK publication, an electric power steering system for vehicles includes an input shaft as a steering shaft connected to a steering wheel, an output shaft interconnected through a universal joint with the input shaft and through a rack and pinion type gear mechanism with a tie rod of driven wheels, an electric motor for supplying auxiliary torque through a reduction gearing to the output shaft, a torque detection mechanism disposed on the input shaft to detect steering torque acting on the input shaft, an electric motor drive circuit for driving the electric motor, and a control circuit for feeding a control signal to the electric motor drive circuit in dependence on a detection signal from the torque detection mechanism.
The electric motor drive circuit above consists of four bipolar type transistors constituting a bridge circuit, and has two functions of controlling the direction of rotation of the electric motor and supplying electric power. Of the bridge circuit, between input terminals there is connected an electric power circuit and, on the other hand, between output terminals is connected the electric motor, and respective bases of the transistors are connected to the side of output terminals of the control circuit. Moreover, the electric motor drive circuit is adapted such that, of the transistors which are arranged in pairs opposing each other in the bridge circuit, either pair has one transistor thereof driven on and the other transistor fed with a PWM signal as a rectangular pulse signal modulated of duration, whereby a voltage of an intended magnitude and polarity is transmitted to the electric motor.
In the electric power steering system with a constitution, when the steering wheel is steered, the output shaft is supplied with adequate auxiliary torque from the electric motor, so that the steering operation is facilitated.
However, in electric motor drive circuits for electric power steering systems for vehicles of such a conventional type as described, a bridge circuit is composed of bipolar type transistors and hence, when driving an electric motor in a PWM manner, oscillatory sounds are generated at oscillation frequencies within an audible range. Moreover, bipolar type transistors are not so high of amplification factor and thus employed by way of a Darlington connection and, therefore, the saturation voltage between collector and emitter is rendered relatively high, resulting in a relatively large power loss. Further, since in general the voltage of a vehicle-mounted battery is as low as 12 V, it is unfavorable to render the saturation voltage high. Furthermore, for the impedance of bipolar type transistors is not so high, there is the problem of an increase in current capacity, with attendant problems such as that the system becomes large in size and thus high in cost.
With such points in mind, the present invention has been achieved to effectively solve problems in conventional electric motor drive circuits for electric power steering systems for vehicles.